Outlook Calendar duplicating entries following an EDIT

The behavior described matches known issues where Outlook classic can show duplicate or near-duplicate entries in shared or moved calendars. There are two relevant patterns and corresponding mitigations in the provided information:

1. Duplicate entries in shared calendars (classic Outlook) Delegates may see duplicate instances of recurring meetings in shared calendars. The documented mitigation is:
   • Remove the affected shared/secondary calendar from Outlook.
   • Re-add the same calendar.
   This resyncs the calendar and typically removes remaining duplicate entries.
2. Duplicate shared calendars when switching modules There is a known issue where users see duplicated shared calendars when switching between the Mail and Calendar modules with REST shared calendars. This has been fixed in Outlook build 16.0.15926.10000. To address this:
   • Check the Outlook build version.
   • If the build is older than 16.0.15926.10000, update Outlook for Microsoft 365 to at least that build so the fix is applied.
3. If duplicates persist after updating If duplicates remain after updating and re-adding the calendar, use one of these approaches depending on whether the items are in a primary or shared calendar:
   • For a primary calendar where duplicates were created by import or move operations:
     1. Switch the calendar view to List (View → Change View → List).
     2. Sort by Subject.
     3. Manually select the duplicate items and delete them.
   • For organizer-side duplicates caused by prior incidents, the Calendar Checking Tool for Outlook (CalCheck) can be configured to detect and remove duplicate items by GlobalObjectID, following the documented steps (set DupGlobalObjectIDs=true and other tests to false, then run CalCheck against the affected profile).

These steps focus on resyncing the calendar (remove/re-add), ensuring the Outlook build includes the duplicate-calendar fix, and then cleaning up any remaining duplicates either manually in List view or via CalCheck where applicable.

References:

• Known issues with classic Outlook Desktop Shared Calendar Improvements
• Duplicate shared Calendar(s) after transitioning between Mail and Calendar modules
• Delete duplicate calendar items
• Outlook shows conflict errors when updating or cancelling meetings